This is not yet mentioned in bpm-platform.xml

<property name="defaultSerializationFormat">
application/json
</property>

So it is rather hard to find how to set the default.

It is even mentioned that the default should be changed (https://docs.camunda.org/manual/latest/user-guide/data-formats/data-formats-in-processes/#serializing-process-variables) - but no hint how to achieve this. A link should be provided